1. Field of the Invention
The present invention relates to the oxidation of sulfur and compounds thereof contained in black liquor as practiced in pulp mills for recovery of sulfur values and the elimination of malodorous emissions and is particularly concerned with a unique system for recovery of high value energy from the heat evolved in such oxidation reaction, which heat would otherwise be dissipated.
2. Prior Art
The oxidation of black liquor is currently being employed in the pulping industry and has been described in numerous publications.
In a typical sequence of operation for cellulosic fiber liberation, as in the manufacture of paper products by the kraft pulping process, the raw wood chips or pieces are cooked or digested in a solution of one or more sulfur compounds. Thereafter the mixture of delignified fibers and treating liquid is sent to a blow tank for pressure reduction, and next transferred to a multiple stage washing facility in which the fibers are separated from the spent chemical-laden wash water filtrate, which filtrate is designated as "weak black liquor". In certain typical plants this black liquor is next subject to concentration in a multiple effect evaporation facility.
Since the spent treating liquid from the digester contains sodium sulfide and other sulfur compounds which are of themselves malodorous or which form hydrogen sulfide and/or other malodorous sulfur compounds released to the atmosphere during the pulp mill operation, it has become the pevailing practice to subject the sulfur-laden spent digestion liquid to oxidation at some selected stage in the sequence, to convert the sulfide and/or other sulfur compounds therein to more stable compounds such as thiosulfates and/or sulfates.
The point in the mill operation sequence at which the oxidation of this so-called "black liquor" is to be best carried out as well as the manner of carrying out the oxidation step, has been the subject of extensive investigation. Thus, among the various proposals advanced, it has been suggested by some proponents that the oxidation step be applied to the weak black liquor from the washing step and prior to concentration. Drawbacks encountered in this procedure led some mills to resort to previous concentration of the liquor and to subject the obtained "strong black liquor" to the oxidation step. Subsequently, it has been proposed, for example in accordance with U.S. Pat. No. 4,058,433, that oxidation be carried out in the weak black liquor preferably at a point between the outlet of the blow tank and the inlet of the pulp washers, preferably employing oxygen in high concentration as opposed to the more usual use of air for the purpose.
Various types of oxidizing reactions have heretofore been used or proposed for use in the oxidation of black liquor. In U.S. Pat. No. 4,058,433, above-referred to, a long narrow counter-flow reactor column is advocated. Other types of oxidizing vessels and arrangements are depicted in U.S. Pat. Nos. 3,362,868; 3,549,314; 3,709,975; and 3,928,531. In accordance with the disclosure in U.S. Pat. No. 3,709,975 certain of the objections and drawbacks of prior black liquor oxidation processes are stated to be avoided by resort to a multi-stage oxidation technique employing oxygen-rich gas, stated to be applicable to both weak and strong black liquor.
Systems and conditions for black liquor oxidation with molecular oxygen are described by Cooper et al in TAPPI 56, No. 6, June 1973 at pages 100 to 103 and in AIChE Symposium Series, Vol. 69, No. 133 at pp 106-115.
In none of the described techniques of the above-cited patents or other known prior art disclosures and practices of the pulp industry, is there found any description of a black liquor oxidation process particularly aimed at maximizing recovery of energy released in the oxidation reaction. Typically in conventional kraft mill recovery systems which employ air or oxygen black liquor oxidation (BLOX) only a fraction (about 20%) of the heat evolved in the oxidation reaction is recovered as high value energy. The remainder is either lost as vented water vapor or recovered as low value heat in the condenser cooling water.